1. Field of the Invention
The present invention relates to a positioning control system for a noncontacting magnetic conveyor system, and in particular, it relates to a system that controls the positioning of a tray in a vacuum chamber on the receiving side when the tray is magnetically conveyed in a contact-free manner between vacuum chambers.
2. Description of Related Art
Rack and pinion mechanisms, roller drive mechanisms, and chain drive mechanisms have hitherto been widely employed when articles are conveyed in vacuum environments. These are called contacting transmission methods, in which the driving force is transferred by contacting action. Such drive mechanisms suffer from pronounced wear at the sliding parts in vacuum environments, where frictional coefficients are high and lubricating oil cannot be used. There has been a problem in that large quantities of dust particles (particulates) occur at the sliding parts. Furthermore, since the frictional coefficient is high, the clearance of the contacting parts must also be increased, and such drive mechanisms have obstructed the precise movement of trays.
However, a strong demand has arisen in recent years for a substantial reduction in the quantities of dust particles adhered to electronic components, typically semiconductor devices. Ideally a drive mechanism in which the occurrence of dust particles has been completely eliminated would be desirable. Various methods have already been proposed for conveyor systems with noncontacting transfer. Of these, the methods with a relatively simple structure are those that use magnetic coupling (referred to as "magnetic conveyor systems" in the following). Proposed magnetic conveyor systems include a linear conveyor mechanism using a helical magnetic circuit (U.S. Pat. No. 5,377,816 or 198/619 XR 414/217) and a magnetic screw used as a feed system in the field of machine tools (Unexamined Published Japanese Patent Application (JP-A) No. H7-280060).
The above-mentioned magnetic conveyor system that uses a helical magnetic circuit to perform linear conveying is equipped with a cylindrical rotary drive member having helical magnetic poles at its surface, and a conveyor path that conveys the moving body. The moving body is equipped with magnetic poles of the same pitch as the pitch of the helical magnetic poles. With such a mechanism, when the rotary drive member rotates, the moving body moves over the conveyor path according to the direction of rotation of the rotary drive member based on the magnetic coupling between the helical magnetic poles of the rotary drive member and the magnetic poles of the moving body.
In a substrate processing system, a tray loaded with one substrate, or with two or more substrates, is conveyed through each process chamber of a plurality of serially arranged process chambers and the substrates are processed in each process chamber. When the abovementioned noncontacting magnetic conveyor system is used to convey trays in such a substrate processing system, noncontacting magnetic conveyor system elements including the above-mentioned rotary drive member and conveyor path must be provided individually for each process chamber. When the tray is conveyed between process chambers in this sort of magnetic conveyor system, the tray must be passed over continuously and smoothly from the magnetic conveyor system element of one process chamber to the magnetic conveyor system element of the neighboring process chamber.
For continuous conveying, when a conveyed article such as a tray is passed over between two independent noncontacting magnetic conveyor system elements, the conveyed article must be stopped at a set position inside the process chamber in the magnetic conveyor system element that has received the conveyed article. Hitherto, pulse motors have been used to perform positioning control with high precision. Positioning with a conventional pulse motor involves determining the time at which the conveyed article is directly sensed by a sensor as the origin of the pulse motor of the magnetic conveyor system in the process chamber on the receiving side.
When a conveyed article is magnetically conveyed in a contact-free manner by magnetic conveyor system elements provided separately in each process chamber, there is no contact between the conveyed article and the rotary drive member when the conveyed article is passed between the magnetic conveyor system elements, and thus vibration (shaking) of the conveyed article occurs in the direction of travel due to inertia. As a result, errors arise in sensing the position of the conveyed article. With the conventional method whereby the conveyed article is sensed by just one sensor, errors (variation) have arisen with respect to the sensed stopping position. Furthermore, there have already been methods whereby a mechanical stopper projects out at the stopping position when a sensor has sensed the conveyed article approaching the stopping position, thereby forcing the conveyed article to stop at the stopping position. However, this has had drawbacks such as dust particles occurring due to collision between the conveyed article and the stopper, and wear occurring on the stopper.